The present invention relates to methods and tools used in failure analysis of integrated circuit (IC) products in the semiconductor industry, and more particularly to a method of mechanically delayering a semiconductor die (also called an integrated circuit chip) in a controlled manner, and an apparatus for carrying out the method.
IC circuits fail due to various physical, chemical or mechanical mechanisms such as electrical overstress, contamination, or wear out. Some failure analysis approaches and procedures require a die to be precisely delayered down to a particular layer to locate such mechanisms. The most well known method of mechanically delayering a die involves progressively abrading the die using a die holder, an abrasive, and a rotatable wheel. The die holder applies the die to the abrasive that is attached to the rotatable wheel.
The die holders currently used are often problematic and limited in their usefulness. These problems and limitations result from instability, imprecision and lack of portability. The prior art has attempted to address these concerns but has fallen short of producing desired and reliable results.
Instability, imprecision and lack of portability adversely affect delayering analysis in several ways. The conventional and most popular method is to secure a die to a die holder, then manually apply the die holder to a rotatable wheel, with the die exposed and sandwiched in-between. The disadvantage of this method is that it introduces inconsistent conditions due to finger pressure variance. Finger pressure variance causes certain portions of the die to be delayered at a faster rate, resulting in non-uniform abrading across the die. Finger pressure variance also significantly changes the abrading angle between the to-be-abraded die surface and the rotatable wheel. This lack of control of the force and directional components results in undesired die surface characteristics, which can be detrimental to delayering analysis. As will be discussed in detail later, delayering the die produces some die surface characteristics that are desirable and some that are undesirable. Lack of control in the delayering process is problematic when the failure mechanism evidence is destroyed from too much delayering.
Another method is to use a delayering attachment with a polishing machine. This method is intended to eliminate finger pressure variance, but instability of the die holder has been known to crack the die or, too often, delayer only one corner of the die. Users continue to use the attachment but therefore often revert to using it with the manual method as described above, rather than with the machine. This reintroduces the finger pressure variance problem.
Lack of portability also contributes to user problems. Lack of portability is the inability of the die holder to be directly used with other failure analysis equipment, for example, an optical microscope, a scanning electron microscope, or a plasma or dry etcher. Thus, prior art devices require the user to detach the delayered die from the holder, and then inspect the die in the appropriate analysis equipment, with another type of holder or without any holder. When more delayering is needed, the user places the die back onto the die holder for more delayering. This introduces undesired variables in the die position, so that if the die is tilted differently or rotated from its position when previously delayered, the abrading produces undesired die surface characteristics, as will be discussed in detail later. The analysis for that particular die is then at an end.
A need therefore remains for a mechanical die delayering method and apparatus that precisely control the abrading angle such that the die is abraded more uniformly. The apparatus also needs to be portable allowing the user to place the die sample in other failure analysis tools without having to remove the die from the die holder
Accordingly, the first objective of the invention is to control the abrading angle. The abrading angle is the angle between the die surface to be abraded and the rotatable wheel. When the die is abraded using a rotatable wheel, a rainbow appears on the die. The rainbow rings on the die can adversely affect visual analysis of particular die circuits, specifically when the rings pass over and obscure transistors of interest. There is no known method of eliminating these rings. The rainbow effect, however, is not a problem when there is a sufficient distance between rainbow rings and the direction of the rings can be controlled. While the required distance will vary with the size of the circuit to be analyzed, a distance of 10 microns will usually suffice; a distance of 1000 microns is ideal. To obtain the maximum distance between rainbow rings, the abrading angle must be decreased to and sustained at a maximum of one degree.
The second objective is to abrade the die more uniformly by decreasing any wobbling that might occur during the delayering procedure. Any wobbling increases the pressure differential. This causes multidirectional rainbow rings, too many of which impair visual analysis of the die. Uniform delayering produces desired unidirectional rainbow rings.
The third objective is to allow the user to place the die sample in other failure analysis tools without having to remove the die from the die holder. The present invention provides that capability by enabling the die to be used intermittently in different tools during the entire delayering process without ever changing its position relative to the die holder.
The present invention meets the above needs and objectives with a new and improved method of delayering a die in a controlled manner, and a new and improved die holder therefor in which the geometry is such that it provides stability and precision to a die delayering process.
In the preferred embodiment, the die holder according to the present invention includes concentric inner and outer cylinders which can be axially adjusted relative to each other. The die which is to be delayered is attached to one end of the inner cylinder, and this end of the cylinder, with the die attached, is positioned just inside the corresponding surface of the outer cylinder so that the die is barely exposed above the adjacent outer cylinder surface. The cylinders and die are then locked in this position, such as by a set screw, following which the assembly is applied against a conventional rotating wheel and abrasive to delayer the die as desired.
The present invention provides precise control, stability, and precision to the process through the ability to carefully control the amount that the die is exposed (the xe2x80x9cexposure incrementxe2x80x9d) and the maximum possible angle (xe2x80x9cwheel anglexe2x80x9d) at which the die surface may be removed or abraded. The dimensions of the cylinders (and particularly the width of the outer cylinder surface) are maintained to keep the wheel angle, ideally, less than one degree. The exact dimensions, of course, will depend upon the actual size of the die, transistor geometries, and the anticipated variations from one sample to another, including variations in the adhesive thickness which holds the die on the end of the inner cylinder. The end result produces the desired delayering or die surface characteristics (the soxe2x80x94called broad xe2x80x9crainbow ringsxe2x80x9d) on a consistent, reliable, and easily produced basis.
The adjustability of the cylinders relative to each other, so that only a very small exposure increment needs to be utilized, also provides for keeping the die holder quite compact relative to die analysis equipment, yet with no loss of precision and performance in the delayering operation. This means that the die can remain attached to the mount during subsequent analysis operations, and then for still further delayering operations thereafter. Not only is this significantly more convenient for the user, but it expedites the diagnostic processes and provides for substantially improved precision in such sequential operations.
It is therefore an object of the present invention to provide a new and improved method and apparatus for delayering dies; wherein the die holder comprises, in combination, an inner member, one surface of the inner member being at least as large as the die which is to be delayered, the outer member having an exteriorly open hole therein, the hole being large enough to removably receive the inner member entirely therein, the hole also being deep enough to provide for receiving the inner member in a position wherein the one surface of the inner member would be beneath the exterior of the hole at a depth substantially as deep as the thickness of the die which is to be delayered, the members being configured to permit the inner member to move inwardly and outwardly through the hole opening, at least one positioner connected to at least one of the members for positioning the inner member relative to the opening, such that, when the die is attached to the one surface, the exposed to-be-delayered die surface is exposed at a predetermined increment just outside the hole opening, the predetermined increment subtending an exposure angle between the highest plane defined by the surface of the outer member around the hole opening and the highest point of the exposed to-be-abraded die surface above the plane, and the predetermined increment defining the exposure angle to be less than substantially two degrees; in which a lock may be configured for locking the inner member relative to the outer member, the lock including a set screw configured for securing the inner member in the hole and a means defining a threaded opening extending from the exterior of the outer member into the hole for adjustably receiving the set screw; in which the die holder inner member may be positioned relative to the outer member such that the exposed die surface is substantially parallel with the highest plane; in which the die holder inner member may be a cylinder, one surface being one end of the cylinder; in which the die holder outer member may also be a cylinder, the hole being substantially the same size as the diameter of the inner cylinder for receiving the inner cylinder for movement longitudinally and coaxially within the outer cylinder; and to accomplish the above objectives and purposes in an inexpensive, uncomplicated, durable, versatile, and reliable method and apparatus, inexpensive to manufacture, and readily suited to the widest possible utilization.